Combining volatiles measurements in fluid inclusions with petrology of ultramafic xenoliths: new insights on the evolution of the West Eifel and Siebengebirge (Germany) Sub-Continental Lithospheric Mantle

Integrating petrography and mineral chemistry data with the determination of volatiles concentration and isotopic fingerprint in fluid inclusions (FI) in ultramafic xenoliths opens a new window on the study of the Sub-Continental Lithospheric Mantle (SCLM). This frontier approach is crucial for understanding nature, evolution and volatiles recycling within the lithosphere, being particularly important in active or dormant volcanic areas, where the signature of the surface gaseous emissions can be compared to that of the deep mantle domains.

Five distinct populations of ultramafic xenoliths brought to the surface in West Eifel (~0.5-0.01 Ma) and Siebengebirge (~30-6 Ma) volcanic fields (Germany) were investigated by combining petrographic and mineral chemistry analyses with noble gases + CO₂ determinations in olivine-, orthopyroxene- and clinopyroxene-hosted FI. Xenoliths from West Eifel are modally and compositionally heterogeneous, as testified by the large forsterite range of olivine, the Cr# range of spinel and the variable Al and Ti contents of pyroxene. Siebengebirge rocks, on the other hand, are quite homogeneous, having mostly refractory composition and reflecting high extents (up to 30%) of melt extraction. Equilibration temperatures vary from 900 to 1180 °C in West Eifel and from 880 to 1060°C in Siebengebirge xenoliths, at comparable oxygen fugacity values. In all xenoliths populations, FI composition is dominated by CO₂, with olivines being the most gas-poor phases and reflecting a residual mantle that experienced one or more melt extraction episodes. The ³He/⁴He ratio corrected for air contamination (Rc/Ra values) in all phases varies from 6.8 Ra in harzburgites to 5.5 Ra in lherzolites and cumulates rocks, suggesting a progressive modification of an original MORB-like mantle signature via interaction with crustal-related components with ³He/⁴He and ⁴He/⁴⁰Ar* signature similar to magmatic gaseous emissions. The mineral phase major element distribution, together with the systematic variations in FI composition, the positive correlation between Al-enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease at increasing temperature, suggest that the SCLM beneath Siebengebirge represented the German lithosphere prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. On the other hand, West Eifel xenoliths bear witness of multiple heterogeneous metasomatism/refertilization events that took place in the German SCLM between ~6 and ~0.5 Ma. According to Ne and Ar isotope systematics, the FI composition in the studied xenoliths can be explained by mixing between recycled air and a MORB-like mantle, being irreconcilable with the presence of a lower mantle plume beneath the Central European Volcanic Province.